This invention relates to franking machines in which a digital printing device is utilised to print a franking impression and in particular to providing security for print data signals which control operation of the printing device.
In known franking machines which are currently in use for franking mail items to indicate that postage charges in respect of those items has been accounted for, the printing of the franking impression is carried out by means of a print drum which carries a print die to print the fixed pattern of the impression and carries print wheels to print variable information. The variable information includes the value of the postage charge for the item and the date of franking the item. The print wheels each have a series of type characters on the periphery thereof and are settable to locate a selected one of the characters in an operative printing position whereby printing of the desired postage charge and date is effected. The print wheels are set to the required positions by mechanisms operated either directly by value setting levers or thumb wheels operated by a user of the franking machine or by stepper motors controlled by electrical drive signals in dependence upon positioning of thumb wheels or operation of keys of a keyboard by a user of the machine. The positions of the levers or thumb wheels are sensed by encoders to provide electrical signals representing a selected postage value to electronic accounting circuits to enable accounting functions to be performed in relation to the selected postage value. In franking machines having a postage meter which operates in a pre-payment mode, a descending register in the meter stores a value of credit available for use in franking mail items with postage charge values and each time a mail item is franked the credit value in the descending register is decremented by the amount of the postage charge value for that item. The postage meter also includes an ascending register to store an accumulated value of postage charge used by the meter in franking mail items and is incremented by the value of postage charge as each item is franked. A further register stores a count of the number of items franked by the machine and is incremented by one each time an item is franked. Accordingly the accounting information stored in the registers provides a record of the postage used in franking mail items and the amount of credit which remains available for use in franking mail items. As is well known in franking machines the electronic accounting circuits are housed in a secure housing to inhibit unauthorised and fraudulent attempts to tamper with the accounting data and with operation of the postage meter. While the surface of the print drum and the selected type characters of the print wheels are exposed, the mechanisms for setting the print wheels are contained within the print drum and extend through an arbour for the drum into the secure housing of the meter. Accordingly the setting mechanisms are protected from unauthorised tampering and the setting mechanisms are so constructed as to prevent changing of the setting of the print wheels by applying force directly to the print wheels. Thus the known mechanical printing devices used in franking machines prevent unauthorised re-setting of the printing elements.
In franking machines currently being developed, it is desired to use non-mechanical digital printing devices such as ink jet print heads or thermal print heads operated directly by electrical signals which define the information both fixed and variable to be printed by the printing device to constitute the franking impression. Such printing heads need to be connected by electrical conductors to the accounting and control circuits of the franking machine in order to receive the electrical signals which are to control and selectively operate the print head. Accordingly it is necessary to prevent unauthorised application of electrical signals on these connecting conductors or to a print data signal input of the printing device resulting in operation of the printing device to print a franking impression.